This invention relates generally to the field of electrically assisted turbochargers and, more particularly, to an improved rotating group with unitary bearing assembly.
The use of turbochargers for boosting charge air in internal combustion engines has been demonstrated as an efficient means for increasing power output and efficiency of the engine. The use of otherwise wasted energy in the exhaust gas of the engine to drive the turbocharger contributes to its efficiency as a boosting device. However, the reliance on exhaust gas contributes to an effect known as xe2x80x9cturbo lagxe2x80x9d in developing power in the engine for acceleration. Boost provided by the turbocharger is dependant on the exhaust energy. Consequently, upon demand for immediate power, the addition of fuel to the charge supplied to the engine occurs first with exhaust energy building slowly increasing energy to the turbine of the turbocharger which, in turn, drives to compressor through the interconnecting shaft to provide boost pressure through the turbocharger in a xe2x80x9cboot strappingxe2x80x9d effect.
Turbo lag can be reduced or eliminated through the addition to the turbocharger of an electric motor that can add energy to the spinning rotor group independent of the actual exhaust energy. The addition of the electric motor allows spin up of the rotor substantially instantaneously to match fuel and boost for the desired power output, with electrical power input being reduced as the exhaust energy becomes sufficient to sustain the necessary level of boost.
The addition of the electrical motor as an integral portion of the rotor group adds to the length and alters the dynamic characteristics of the rotor. To maintain efficient operation of the turbocharger, alterations in the bearing system and rotor are, therefore, required. Improved floating journal bearing systems such as that disclosed in U.S. Pat. No. 5,857,332 entitled BEARING SYSTEMS FOR MOTOR-ASSISTED TURBOCHARGERS FOR INTERNAL COMBUSTION ENGINES, issues on Jan. 12, 1999, provide one approach to achieving such alterations. However, it is desirable to create a rotor and bearing system with increased efficiency and robustness to accommodate the electrically assisting motor system in the turbocharger.
A turbocharger incorporating the rotor and bearing system of the present invention includes a turbine wheel carried in a turbine housing to receive exhaust gas from an internal combustion engine. The turbine wheel is attached to a shaft by intertial welding or similar means and incorporates a hub portion which includes a piston seal engaging the turbocharger center housing. A compressor impeller is attached to the shaft at the opposite end from the turbine and carried within a compressor housing which receives the inlet air and includes a diffuser and volute to carry boosted air from the compressor to the engine intake system. A bearing portion of the shaft adjacent to the turbine hub includes two bearing lands separated by a relieved section. The lands interface with a pinned semi-floating unitary bearing carried in a bearing bore in the center housing. The unitary bearing includes a first thrust bearing face engaging the turbine hub. A permanent magnet for the electric motor is carried by a spacer collar which is received over a collar portion of the shaft extending from the bearing portion opposite the turbine hub and the unitary bearing incorporates a second thrust face engaging the spacer collar. The spacer collar also includes a piston seal engaging the center housing. The spacer collar engages the compressor impeller to maintain the fixed length of the rotor.